A common application of transparent self-adhesive films such as, for example, tesa® Multifilm® from tesa AG, Hamburg (D), or Scotch® Magic from Minnesota Mining and Manufacturing Co. Inc., St. Paul (USA) is the temporary fastening of very light, usually flat articles such as, for example, very small pictures, photos, postcards, posters, drawings, decorations, etc., in the home, work, and office sectors. Self-adhesive films are often used here as alternatives to drawing pins and the like. Advantages of self-adhesive tapes as compared with the latter fastening means include their simple and unhazardous handling (no risk of injury), their substantial invisibility (owing to the high transparency of the self-adhesive films), the possibility of fixing to solid surfaces which needles cannot penetrate, and the possibility of redetaching a once-performed bond from sufficiently solid substrates without residue or destruction, by peeled removal of the adhesive film. The peel removal of self-adhesive films is an action which is typically known even to non-experts. A disadvantage of a fixture effected with self-adhesive films is that the said films are often not redetachable from low-strength substrates such as wallpapers, paper, paperboard, painted walls, etc., without at least partial destruction of the substrate.
Numerous PSA systems and self-adhesive tapes have been specially developed in order to allow bonds to be redetached without residue or destruction even from substrates which are in some cases of relatively low strength. One direction of development in this context has pursued the production of reversibly peel-removable single-sidedly or double-sidedly pressure-sensitively adhesive self-adhesive tapes. Typical examples of corresponding products make use, among others, of:    1. PSAs based on acrylate copolymers which are prepared by means of suspension polymerization (bead polymerization). Exemplary embodiments are described in WO 96/18701 A1, “Removable pressure sensitive adhesive and article” and the literature cited therein. Typical market products include “GELTAC Microsphere Adhesive Tabs” from PCI, Syracuse, N.Y. (USA) and “Postertape” from verkerke (Netherlands).    2. Self-adhesive tapes which are partly coated with PSAs and which acquire reversible peel removal properties by virtue, inter alia, of the nature of the partial coating. For exemplary embodiments see EP 0 149 135 A1 “Self-adhesive flat items removable without a trace” and EP 0 328 925 A1 “Self-adhesive flat items”.    3. PSAs based on polyurethanes. Market products include “Doppio” from Oxford (GB), “Mount No. 30” from Cemedine or Tree's Klebe Pins from Plus AG.
Redetachable, reusable PSAs based on polyurethane are described in, for example, U.S. Pat. Nos. 6,040,028 A or 5,102,714 A. Double-sided adhesive tapes with an in elastic backing, comprising one or more redetachable, washable, reusable PSAs based on polyurethane, are known from, for example, JP 08 188 755 A1 or JP 06 279 741 A1.    4. Other PSA systems, for example those based on solution-polymerized acrylate copolymers, natural rubber/resin blends, ethylene-vinyl acetate copolymers, to name but a few, whose profile of adhesive properties is controlled in such a way as to allow the production of adhesive tapes having very low peel forces. Market products include, “Scotch transparent mounting squares” from Minnesota Mining and Manufacturing Co. Inc. and “Pritt on&off” from Henkel.    5. Plastically deformable PSA products, which can be portioned and shaped like modelling clay and of which examples include those known under the brand name “Blu-Tac” (Bostik), “UHU tac patafix” (Uhu GmbH) or “Pritt Buddies” (Henkel).
On the basis of the aforementioned PSA systems it is possible to produce single-sidedly or double-sidedly pressure-sensitively adhesive self-adhesive products which as a consequence of their low peel forces can be redetached without residue or destruction simply by peeling even from substrates which are in some cases of relatively low strength. Through the use of double-sidedly pressure-sensitively adhesive self-adhesive tapes it is possible in the same way to redetach flexible materials such as, for example, posters, papers, and polymer films, etc. without residue or destruction even from substrates which are in some cases of relatively low strength.
The abovementioned self-adhesive articles, however, have numerous disadvantages:                In all cases in which it is possible to redetach the self-adhesive products without residue or damage from low-strength substrates such as, for example, wallpaper, paper, paperboard, painted walls, etc., by peeling it is possible only to realise extremely low bond strengths. It should be borne in mind here that the forces exerted on the bond substrate when detaching self-adhesive tapes are transmitted along a very narrow line and consequently, even in the case of low peel forces, considerable peel stresses (normal stresses) can act on the bond substrate, and may give rise at least to partial damage of low-strength substrates along this narrow line.        In the fixing of two rigid adherends, owing to their lack of flexibility it is impossible to redetach them without residue or destruction by peeling. Any attempt to part the bond by cleaving the adhesive bond is typically accompanied by such high cleavage forces (normal forces) (acting orthogonally with respect to the adherends) that at least partial destruction of one of the adherends in many cases cannot be ruled out.        The abovementioned products typically show an increase in the peel force required for their redetachment over time. Thus, for example, for Pritt on&off (Henkel) and, “GELTAC Microsphere Adhesive Tabs” from PCI, Syracuse, N.Y. (USA), it is observed that freshly adhered poster papers can be redetached without residue or destruction from painted woodchip wallpaper. If, however, the posters have been adhered to the same wallpaper substrate for several weeks, which corresponds to a typical application time, the peel forces, which are now markedly higher, mean that the posters and the wallpaper are often severely damaged in the course of peel detachment. Residueless and non-destructive detachment from sensitive substrates is therefore possible only to a limited extent, and in many cases not at all.        A serious disadvantage of many of the abovementioned self-adhesive products is their characteristic of causing grease strikethrough on paper and paperboard substrates in particular and hence of irreversibly altering the optical and mechanical properties of such substrates. This is particularly annoying when optical alterations which occur permanently adversely change the aesthetics of an article to be bonded or of the substrate. This disadvantage occurs particularly for all existing polyurethane-based PSAs.        A further disadvantage of aforementioned peel-detaching, non-washable self-adhesive tapes is that such tapes typically, after use or after, for example, soiling (as a result, for example, of finger contact or contact with soiled surfaces), exhibit a marked reduction in their adhesion and hence typically cannot be reused without a marked loss of performance.        
Self-adhesive tapes which are redetachable by peeling without residue or destruction can therefore be utilized for a multiplicity of fastening tasks, particularly for very lightweight, flat articles, but at the same time, as demonstrated, have a significant number of disadvantages.
A second direction of development in PSA systems and self-adhesive tapes for the residueless and non-destructive redetachment of adhesive bonds even on substrates of relatively low strength is pursuing the production of elastically or plastically highly extensible (strippable) self-adhesive tapes which can be redetached without residue or destruction by extensive stretching in the bond plane.
Typical examples are known from publications including U.S. Pat. No. 4,024,312 A, DE 33 31 016 C1, WO 92/11332 A1, WO 92/11333 A1, DE 42 22 849 A1, WO 95/06691 A1, DE 195 31 696 A1, DE 196 26 870 A1, DE 196 49 727 A1, DE 196 49 728 A1, DE 196 49 729 A1, DE 197 08 366 A1, WO 99/31193 A1, DE 198 20 858 A1 and DE 100 55 942 A1. They are frequently employed in the form of adhesive tape strips (adhesive sheet strips), which preferably have a non-adhesive grip tab region, starting from which the detachment operation is initiated. Specific embodiments of aforementioned self-adhesive tapes are described in, for example, in DE 44 28 587 A1, DE 44 31 914 A1, WO 97/07172 A1, DE 196 27 400 A1, WO 98/03601 A1, DE 196 49 636 A1, DE 198 42 864 A1, DE 198 42 865 A1 and DE 199 38 693 A1. Advantages of self-adhesive tapes redetachable without residue or destruction by extensive stretching are to make these tapes such that the stretching which is performed in the course of detachment significantly reduces the adhesion of the PSA. As a result, the detachment forces (normal stresses) which act on the adherends orthogonally to the bond surface in the course of detachment are very low. This makes it possible to redetach adhesive tapes, even those possessing very high peel strengths, and even from highly sensitive substrates, without residue or destruction. A particular advantage in comparison to the above-described adhesive tapes which are redetachable by peeling is the fact that even planar bonds between rigid adherends can be redetached without residue or destruction.
The self-adhesive tapes described in aforementioned documents and redetachable by extensive stretching in the bond plane have been designed, however, for the bonding of articles of medium to heavy weight, and so must have high bond strengths and hence also such high peel strengths that reversible peeling from the majority of substrates is impossible. A further disadvantage is that the redetachment by extensive stretching only functions reliably without damage when the adhesive sheets have been cut essentially into strip form or into an at least elongate form and when the extensive stretching takes place in the lengthwise direction of these forms. Moreover, generally speaking, a non-adhesive grip tab region is necessary, particularly when bonding is to take place onto substrates of relatively low strength, on which even only slight peeling in the initial region of the bond section leads to damage to the substrate.
It is apparent that, particularly for the fixing of small articles, adhesive tapes which have very low stripping forces would be advantageous in order to allow easy and simple detachment, including for example by children.
A very large proportion of the fixtures desired in the home and office sectors relate, as described above, to small and very small articles, such as, for instance, the fixing of very small pictures, postcards, photos, drawings, and, typically, articles attached to notice boards, for the fastening of which, conventionally, simple self-adhesive films, drawing pins and the like frequently offer sufficient bonding performance. For aforementioned applications, the self-adhesive tapes on the market hitherto and described in abovementioned documents, which are redetachable by stretching in the bond plane, are thus not very suitable. Disadvantages are that they are inconvenient to handle and that they cannot generally be used more than once, either since as a result of the extensive stretching they undergo irreversible plastic deformation or become screwed up adheringly in such a way that straightening them out is no longer possible.
It is apparent, furthermore, that the operation of detachment by extensive stretching in the bond plane is for many users not an action which has been mastered in the same way as for the parting of a bond by peeling. This frequently turns out to be a disadvantage, since in numerous instances the parting of the adhesive bond must take place by a stretching precisely parallel to the existing bond plane in a preferential direction, in order to ensure tearing of the adhesive strips and a residueless and non-destructive parting of the adhesive bond.
A disadvantage of aforementioned self-adhesive tapes which occurs here as well is that such tapes, following use or, for example, soiling (for example, by finger contact or by contact with soiled surfaces), display a marked reduction in their adhesiveness and hence cannot be used again without a marked loss of performance.
It is an objective of the present invention, accordingly, to provide single-sidedly or double-sidedly pressure-sensitively adhesive sheets, preferably in the form of diecuts or cut shapes, which are suitable for redetachable bonding without residue or damage of small, lightweight articles having a low-strength surface, such as, for example, a paper surface, and which no longer have the depicted disadvantages of the prior art, or not to the same extent.
In particular, it ought to be possible to part the bonds without residue or damage even when peeling is not possible, owing to the geometric circumstances in the area surrounding the bond site, this being the case particularly when rigid adherends are fixed to one another or when the bond has already existed for a relatively long time, several months for example, and consequently the peel force, depending on the nature of the bonded substrate, may possibly have risen sharply in comparison to the peel force in the case of a fresh bond. The redetachment without residue or damage should be realizable always easily and in any direction, irrespective of the external form of the PSA sheet. It should be possible to reuse the PSA sheet in any case, even when the PSA of the sheet has been soiled or when the sheet has become screwed up.